The ISO 13406-2 method to measure the monitor’s response time as the total time necessary to change the state of a pixel from pure black to pure white and back again brings but very little information about the real performance of the monitor and easily misleads the user. Today we are going to reveal all secrets about this parameter and discuss the response time compensation that affects the quality of dynamic images on the screen of 7 monitors from ViewSonic and Samsung.

But enough of others’ delusions, let’s learn more about the technical aspects of response time compensation instead. Obviously, the non-linear and non-monotonic dependence between the response time and the initial and final brightness of the pixel make it necessary for the monitor’s electronics to choose the compensation value individually, depending on the current and final state of the pixel. To do this, the RTC block includes a frame buffer that stores the previous frame. When the next frame arrives, it is compared with the contents of the buffer and the compensation value is chosen for each pixel whose brightness has changed.

Besides the above-described effect of “slow crystals”, there is also one more response-time-affecting factor – the electric capacity of the cell changes when the crystals within it are turned round. The cells of an LCD matrix are not constantly connected to a power source. The required voltage is set in them by a short impulse at the refresh frequency and is maintained after the impulse because each cell is a capacitor. Unfortunately, the capacitance of this capacitor is not constant, but depends on the position of the crystals. Suppose the voltage U0 was initially applied to a cell, but a new frame has changed it to U1. The cell capacitor took the charge Q=U1*C1 where C1 is its capacitance at the moment. The crystals begin to turn round and the capacitance changes and becomes C2 by the moment the next frame arrives. Since the charge remains the same, the voltage necessarily changes along with the capacitance, according to the formula U1*C1 = U2*C2. And the voltage changes in such a way that it now impedes the turning of the crystals. When the next frame arrives, the voltage U1 is again set on the cell and the speed of the crystals changes again, too (for the sake of simplicity I suppose that the cell’s color hasn’t changed between the frames). This step-like process can be illustrated by a diagram that shows how the brightness of a pixel is changing in time:

This graph (I took it on a BenQ FP737s-D) shows a long horizontal stretch that ends only with the arrival of the next frame. The level of that stretch is not high, but in some cases it may result in a barely visible afterglow behind moving objects. The response time compensation concept helps here, too – the cell must get such a voltage at the beginning of the first frame that it automatically reached the necessary brightness level by the end of the frame as the result of the change in the cell capacitance.

It is impossible to achieve absolute accuracy in Nature and, moreover, the monitor developers must also try to make the cost of the finished product low (the new models wouldn’t be so interesting if their price turned to be two or three times that of the older ones). The requirement for the new electronics with RTC not to be much more expensive than the older one limits its functionality, particularly the accuracy of the compensation impulse.